Assignment Statements and Input

Hello! I'm Saavi, and I'm so glad you're here. Let's dive into how we make our programs remember and interact with things.

The Assignment Operator: Your Program's Memory

Think of a variable as a labeled box. When you declare a variable, like int score;, you're creating an empty box with the label "score" on it, and you're telling the computer that only integers can go inside.

But an empty box isn't very useful. We need to put something in it. This is called assignment.

The assignment operator in Java is the single equals sign (=). It tells the computer: "Take the value on the right and store it in the variable on the left."

int score;      // Declares an integer variable named 'score' (an empty box)
score = 100;    // Assigns the value 100 to 'score' (puts 100 in the box)

The first time a variable is given a value, we call that initialization. You can declare and initialize in one step, which is very common:

int score = 100; // Declares AND initializes the variable 'score'

The Golden Rule: Right-to-Left

Always evaluate the right side of the = first. The computer calculates everything on the right until it becomes a single value. Then, and only then, does it store that single value in the variable on the left.

Consider this:

int newScore = score + 5 * 2;

1. The computer looks at the right side: score + 5 * 2.
2. It knows score is 100. So, it's 100 + 5 * 2.
3. It follows the order of operations (PEMDAS): 100 + 10.
4. It evaluates this to a single value: 110.
5. Now, it takes that 110 and stores it in the newScore variable.

The value of the variable on the left is always overwritten. If score was 90 before, after score = 100;, the 90 is gone forever, replaced by 100.

Type Compatibility is Key

Your labeled box has rules. You can't put a basketball in a shoebox. Similarly, you must assign a value of a compatible data type.

int userAge = 21;         // Good! 21 is an int.
double price = 19.99;     // Good! 19.99 is a double.
// int tax = 8.25;        // BAD! This will cause an error. 8.25 is a double, not an int.

For object references, you can assign a new object or a special value called null. null is a keyword that means "this reference doesn't point to any object." It's like having a label for a box, but the box itself doesn't exist yet.

String name = "Marcus"; // 'name' refers to a String object with the value "Marcus"
name = null;            // Now, 'name' refers to nothing at all.

Getting User Input with the Scanner Class

So far, all our data has been hard-coded by us, the programmers. But what if we want to write a program that calculates the tip for a bill at a restaurant in Seattle? The bill amount changes every time! We need to get that information from the user.

This is where the Scanner class comes in. Think of it as a tool that listens for input from the keyboard.

To use it, you need to do two things:

1. 1
   Import it

   At the very top of your file, you need to tell Java where to find the Scanner tool. import java.util.Scanner;
2. 2
   Create it

   Inside your main method, you need to create a Scanner object and connect it to the standard system input (the keyboard).

import java.util.Scanner; // Step 1: Import

public class Greeter {
    public static void main(String[] args) {
        // Step 2: Create a Scanner object
        Scanner keyboard = new Scanner(System.in);

        System.out.println("Hello! What's your name?");

        // Use the Scanner to read the user's next line of text
        String userName = keyboard.nextLine();

        System.out.println("Nice to meet you, " + userName + "!");

        System.out.println("How old are you?");

        // Use the Scanner to read the user's next integer
        int userAge = keyboard.nextInt();

        System.out.println("You'll be " + (userAge + 10) + " in a decade!");

        keyboard.close(); // Good practice to close the scanner when you're done.
    }
}

The Scanner class has different methods to read different types of data:

• nextLine(): Reads a whole line of text as a String.
• nextInt(): Reads the next whole number as an int.
• nextDouble(): Reads the next decimal number as a double.

By combining user input with assignment statements, you can create dynamic programs that respond to user needs. You ask for information, store it in variables, and then use those variables to calculate, decide, and produce new results. This is the foundation of interactive software.

Let's walk through a couple of examples to make these concepts solid.

Ready to try it yourself?

1. 1
   Temperature Converter

   Write a program that asks the user for a temperature in Fahrenheit (as an integer). Convert this temperature to Celsius using the formula C = (F - 32) * 5 / 9 and print the result.

   • Hint: Be careful with integer division! To get an accurate decimal result, you might want to use 5.0 / 9.0 in your formula and store the result in a double.
2. 2
   Simple Mad Libs

   Create a program that asks the user for a name (String), an adjective (String), and a verb (String). Store these in three different variables. Then, use them to print a silly sentence like, "[Name] [verb]s to the [adjective] store."

   • Hint: You'll need to create a Scanner and use the nextLine() method three times to get the user's words.